Manufacturers of finished aluminum products, such as sheets, rods, windows, doors, etc., use etchant solutions for surface passivation, or for preparing a surface to receive a coating of paint or other coating material. Chromic acid may be used alone as the etchant solution or, more commonly, used with phosphoric acid or other additives, such as zinc, molybdenum, HF, or HNO.sub.3. The spent etchant solutions are typically treated for chromium removal by acidifying the solution to a pH 2 to 4 and reducing the chromium from Cr(+6) to Cr(+3) with sodium bisulfite or another chemical reductant or by electrolytical reduction. The Cr(+3) is then precipitated as chromium hydroxide by adjusting the pH of the solution to about 6 or above with NaOH, KOH, MgO, CaO or combinations thereof. The precipitated chromium hydroxide is then recovered by settling and decantation, centrifugation, and/or filtration. The resulting chromium-containing material is typically produced as a wet filter cake (50-75% moisture). The waste may also be produced in the form of solutions, sludges, slurries, dried solids, and the like. The waste contains predominantly chromium, aluminum, calcium and phosphates and minor amounts of iron, zinc, magnesium, fluorides, sodium, potassium, etc.
This waste material is designated as a hazardous waste under code FO19 by the U.S. Environmental Protection Agency (EPA), mainly because of its chromium content, and its disposal is regulated by federal, state, county, and city ordinances. In the past, the sludge was typically disposed in landfills after dewatering. However, this practice is coming under greater scrutiny by the regulating authorities, primarily due to the presence of the chromium hydroxide which is water leachable under mild acidic conditions. To prevent any leaching of the metal hydroxide, secure and chemically maintained land disposal sites are required. Additionally, regulations may require that the chromium be immobilized or fixated before landfilling. Thus, the disposal costs associated with chromium-containing wastes are high and are continually increasing. The disposer also remains liable for the waste material as long as it can be traced. These and other factors add to the incentives to find safe and economical means for disposal of the waste.
Accordingly, it is advantageous from an environmental and economic standpoint to reduce the chromium content of the material prior to disposal and recover the chromium in a marketable form. In U.S. Pat. No. 4,954,168, a method is disclosed for the recovery of chromium from waste material as synthetic chromium oxide concentrate. But the marketability of this concentrate is somewhat limited due to one or more objectionable impurities. For example, the phosphorous content of this concentrate is objectionable to producers of steel alloys, and the sodium content is objectionable to producers of refractory materials.
The purpose of this invention is to provide a method for recovering chromium, and optionally aluminum, from waste material as useful products of commerce, while reducing the toxicity of the waste to facilitate its disposal. Of great significance, the present invention permits the recovery of chromium in a form which is sufficiently pure to be usable in a broad number of applications.